I am pretty sure the Schematic is OK, just I am confused as to how to calc the gain since it is a screwy topology. Broskie's explanation was pretty clear as to the advantages but "thin" on the math.

I looked at the cornet but there are SO many poles in the B+ I am assuming it is part of the EQ network, Looks too complicated for what I am trying to do. (I have very little vinyl and play it rarely) I just have a lot of extra mA's in the PS for the preamp and figured I would put it to use.

Not to "HIJACK" my own thread but I have been pondering a question maybe this is an appropriate forum to ask it.

Here's the scenario:

I am laying out my next few projects based upon the eventual system configuration that I want to achieve. I am currently out of work so funds are limited but I have a LOT of free time.

I am preparing a "listening room" in my basement, which will also double as my home office and computer room.

Since I spend a LOT of time on the computer and love all kinds of music my collection of MP3's has grown to the point that I have needed to upgrade the hard drive 3 times (120+GB now).

I have a very nice sound card but it has VERY low output. Thus the need for the preamp even though my 6V6 PP "workhorse" amp has plenty of gain it still can't drive the power section to beyond about 1/2 full output.

So (after the long winded explanation) I would like to achieve the following.

Using the 6V6PP amp I already have (about 8W/Channel) and another PP amp (TBD) I would like to "Bi-Amp" my Yamaha's

I addition I would like to add a "subwoofer" or "bass channel" for this I will tackle the challenge of getting about 20 Watts from 829B's in SET. Since I plan for this to be about 140hz and down @ 6db/octave and most sound below 400 is non directional I plan for it to be "Mono" with a single driver.

SOOO....

I need to "SUM" the L/R channels for this "mono" LP filter.

Oops Forgot to mention I plan to also insert an "Active Tube Crossover" between the preamp and the power amps.

Maybe I am missing something but I can't get a "grasp" on how to sum the signals without messing with the "stereo" separation of the signals for the rest of the system.

IE simply "connecting" them together would work backwards also?
What I am getting at is I need some sort of "buffer" correct?

here's a good thread on why not to use a mono sub (it' wasn't in the loudspeaker forum afterall, sorry). it's a mastering forum - those guys who use high end system to evaluate and/or tweak records before they're put out to the public :

I have read (can't remember where but published and reputable sources) that typical human hearing cannot differentiate sound sources below about 400hz this is also exponential to the octave so at 200hz were about 4 times less likely to identify the source location.
Example: Aside from left right front or back can you tell WHERE the "boom" of thunder came from? Maybe the "crack" (higher frequency) but the BOOM is hard to tell.

So making the assumption that the track was recorded and is being played back in "Acoustically perfect and identical rooms" (just to eliminate room acoustics) I see no reason why a "mono" sub woofer would be an issue. I agree with some that "summing" electrically the 2 channels will result in some "lost" information, ie something that "cancels" from L/R.

BUT, that would have to have been recorded directly to the track or with "stereo" mikes. Either way had I PERSONALLY been in that room the sounds would have "cancelled" before they got to my ears anyway and if they didn't I would not know whether they were from the left or right.

Since that is a SMALL portion of the sound information I can live with it being "lost" altogether in exchange for not pushing my speakers to the limit with long excursion @ low frequency. Remember no drivers suspension is perfect. Therefore, excessive low frequencies into the driver can "muddy" up the sound.

Also, relatively speaking (no pun intended) I can afford a much higher sensitivity in a woofer than I can the full range drivers. This leaves me with more room to play with a lower power "sub amp"